This invention is directed to a thermal actuator and, more particularly, to an electro-thermal actuator having an internal fluid reservoir in the variable volume chamber. Moreover, the present invention also is directed to an improved electrical feed through connection for a thermal actuator having an internal electric resistance heater and to an arrangement for reducing the time required for recycling a thermal actuator.
A typical fluid actuator usually has a variable volume chamber that expands and contracts in response to pressure increases and decreases therein. The changes in chamber volume may be readily converted to a linear or other mechanical output to perform work on an external device. In one type of fluid actuator an external fluid supply is provided to the chamber for increasing and decreasing the pressure therein, and in another type of fluid actuator the chamber is sealed and contains an expansible and contractible working medium. In the latter type fluid actuator changes in the chamber pressure may be effected by applying to the working medium energy, such as thermal energy in the form of heat or cold. The thermal energy may be applied via part of the actuator main body, casing, or the like, for example, through a heat conductive window; or the thermal energy may be applied, for example, by a heater located in the variable volume chamber. Moreover, in the latter type actuator, which will be referred to hereinafter as a thermal actuator regardless of the energy input, various types of working media may be used, such as, for example, liquids that undergo a liquid to gas phase change upon application of heat, metal hydrides that out gas in response to the application of heat, thermally responsive waxes, or any other material that will effect a change in the chamber pressure in response to application of energy thereto.
In thermal actuators that use a liquid working medium which undergoes a liquid to gas phase change upon application of heat, it is desirable to have a supply of liquid at the heated area to avoid excessive temperatures thereat and, particularly, in those thermal actuators that have internal electric heaters the liquid supply cools the heater and reduces the possibility of burn out. Also, since thermal actuators may experience some leakage, for example by permeation through the extensible member, which may be a diaphragm, the reservoir liquid will ensure an adequate supply of liquid to the heater over a relatively long life of the actuator. An absorbent or wicking-type reservoir that provides a fluid supply to the heater of the thermal actuator is disclosed in U.S. Pat. No. 3,805,528.
Moreover, for those thermal actuators that include an internal electric resistance heater or the like, it is desirable to ensure the integrity of the heater and the electrical connections thereto while at the same time maintaining the sealed condition of the variable volume chamber.
Usually a thermal actuator may be energized to a maximum output or outstroke condition in a relatively short time compared to the time required for recycling or cool down of the actuator to an instroke condition. Therefore, it is also desirable to reduce the required recycling time to improve the operational characteristics and use options of a thermal actuator.